1. Field of the Invention
The present invention relates to an optical disc, such as a DVD, to be a medium for storing digitized video and audio works and the like, such as movies and music. In addition, the present invention also relates to an optical disc device which reads information recorded on an optical disc.
2. Description of the Related Art
=Overview of DVD Standard=
As is generally known, there is a DVD (digital versatile disc) as an optical disc for storing digital images. The DVD has been widely used all over the world mainly for storage and distribution of movie contents (publication of digital products). This DVD is a standard worked out by the DVD forum, which has been made public as a DVD standard (DVD Book) (see www.dvdforum.org) and also specified by International Standards and JIS. Here, a brief description will be given according to the International Standard of a 120-mm DVD-ROM, IS0/IEC16448, which is one of the DVD physical standards.
As to the 120-mm DVD-ROM, there are four types of discs in total, including a single-sided specification with a single recording layer, a single-sided specification with dual recording layers, a double-sided specification with a single recording layer, and a double-sided specification with dual recording layers. For storage and distribution of contents such as movies, the two types of the single-sided specifications are mainly used. As to a disc capacity, a single-sided single-layer disc has a capacity of 4.7 GB, and a single-sided dual-layer disc has a capacity of 4.27 GB on each layer (8.54 GB per disc).
=Single-Sided Single-Layer DVD=
FIG. 1 shows a relationship between a basic structure of a single-sided single-layer DVD disc 10 and an optical head. As is well known, the DVD disc 10 has a structure in which two disc substrates, each having a thickness of 0.6 mm, are attached to each other. One of the substrates is a signal substrate 11, and the other is a dummy substrate 14. The two substrates are attached to each other by means of an adhesion layer 24 so as to dispose a recording layer 20 therebetween. Normally, these substrates are made of a plastic material called polycarbonate by use of an injection molding machine.
Note that image information, data information and the like are recorded on a spiral track in the form of an embossed pit in the signal substrate 11. A red laser 30 (wavelength: 650 nm) which reads information on the recording layer is condensed by an objective lens 35 (NA: 0.6) and focused onto the recording layer 20 through an optically transparent layer 13 in the signal substrate 11.
Each of FIGS. 2A and 2B shows a position of the recording layer viewed from an incident surface 12 of the single-sided single-layer disc. FIG. 2A shows the case of Normal single layer, which is conventionally used, where the center value of the thickness of the optically transparent layer 13 is 600 μm, and the recording layer is disposed at a distance in a range from 570 μm to 630 μm inclusive from the incident surface. This value is set in consideration of a spherical aberration of the objective lens 35. Among the cases of single-sided single layer disc, a case with Thin single layer has been recently added as a DVD standard (see www.dvdforum.org or DVD Book), if the case, as shown in FIG. 2B, complies with that the center value of the thickness of the optically transparent layer 13 is at 565 μm and that the recording layer is disposed at a distance in a range from 550 μm to 580 μm inclusive from the incident surface.
=Single-Sided Dual-Layer DVD=
FIG. 3 shows a relationship between a basic configuration of a single-sided dual-layer disc 15 and an optical head. As is well known, this disc includes a first recording layer 21 and a second recording layer 23. These two recording layers are accessed from one side of the disc, and a signal can be reproduced. As shown in FIG. 3, when viewed from a light incident surface 17, on the other side of an optically transparent layer 18, the first recording layer 21 is located nearer, and the second recording layer 23 is located farther. Accesses to the respective recording layers are realized by causing a lens actuator to move an objective lens 35 so that the objective lens 35 can make an interlayer jump.
Manufacture of the dual-layer disc described above has a main feature that the disc can be manufactured in almost the same manner as the single-sided single-layer disc. First, by use of an injection molding machine, a signal substrate 16 on which the first recording layer 21 is formed, and a signal substrate 19 on which the second recording layer 23 is formed are separately prepared. Next, a semitransparent film is attached to the first recording layer 21, and a highly reflective film is attached to the second recording layer 23. Thereafter, the both substrates are attached to each other by means of an intermediate layer 25 in a manner that the recording layers face inside. Thus, the single-sided dual-layer disc is completed.
FIG. 4 shows positions of the recording layers viewed from the incident surface 17 of the single-sided dual-layer disc. In consideration of a spherical aberration of the objective lens and a crosstalk between the recording layers, the position of the first recording layer 21 is limited in a manner that its distance from the incident surface is 550 μm at the smallest, and the position of the second recording layer 23 is limited in a manner that its distance from the incident surface is 640 μm at the largest. In addition, a distance between the two layers (a thickness of the intermediate layer 25) is limited as to be 55 μm±15 μm (from 40 μm to 70 μm inclusive). The thickness of this intermediate layer 25 is usually that of an adhesion layer formed when the two substrates are attached to each other. In actual manufacturing, the thickness of the intermediate layer 25 is determined in consideration of an attachment accuracy and a molding accuracy of the signal substrate 16.
=Reflectivity of Recording Layer and so Forth=
Meanwhile, reflectivities of the recording layers are determined as follows.
Single-Layer Disc: 45 to 85% (PBS present) or 60 to 85% (PBS not present), circularly polarized light, and
Dual-Layer Disc: 18 to 30% (PBS present) or 18 to 30% (PBS not present), circularly polarized light
Moreover, information indicating a reflectivity of a disc is located at b29 among 4-byte ID (identification data) in a data frame. Here,
0b: the case where the reflectivity is greater than 40% (PBS present), and
1b: the case where the reflectivity is less than or equal to 40% (PBS present)
Other than the above, the ID includes the following.
Area type: b27-b2600bIn the Data area01bIn the Lead-in area10bIn the Lead-out area11bIn the Middle areaData type: b250bRead-only data1bOther than read-only dataLayer number: b240bLayer0 of DL discs or on SL discs1bLayer1 of DL discs
Moreover, as to information indicating whether the disc includes one layer or two layers, Disc structure is defined by (BP2) in a control data zone, and b6-b5 shows the number of recording layers.
00bSingle01bDualOthers:reserved
Furthermore, a capacity per one layer differs between a single-layer disc and a dual-layer disc. This is because of a difference in a linear recording density. The recording density is defined in (BP3), and b7-b4 shows the linear recording density as follows.
0000b:0.267 μm (linear recording density of a single-layer disc)0001b:0.293 μm (linear recording density of a dual-layer disc)=Next-Generation DVD=
Meanwhile, as has been recently well publicized, there has been proposed a HD DVD whose density is increased to at least 3 times as high as a DVD by use of a blue-violet semiconductor laser (hereinafter referred to as a blue-violet laser) in order to satisfy a desire to store HD (high-definition) images in one disc. Moreover, standardization of the HD DVD has been underway in the DVD forum (See www.dvdforum.org. The HD DVD has not yet been commercialized.)
The HD DVD has the same disc structure as that of the conventional DVD. As to a capacity thereof, a single-sided single-layer disc has a capacity of 15 GB, and a double-sided dual-layer disc has a capacity of 30 GB. These large capacities are realized by new technologies such as shortening of a wavelength of a laser beam, increasing of a NA, an improved modulation method, and a new signal processing (PRML: partial response and most likelihood).
FIG. 5 shows a relationship between a basic structure of a single-sided dual-layer HD DVD disc 40 and an optical head. The HD DVD has a different spherical aberration, a different coma aberration by a tilt, and the like. This is because a laser beam 60 which reads information from the disc is shortened from a red laser beam (650 nm) to a blue-violet laser beam (405 nm), and a NA of an objective lens 65 is increased from 0.6 to 0.65. Thus, an actual HD DVD disc is slightly different from a DVD in positions of first and second recording layers 51 and 53, a thickness of an intermediate layer 55, and the like.
FIG. 6 shows the positions of the recording layers of the single-sided dual-layer HD DVD disc when viewed from an incident surface 46. Along with the shortened wavelength and the increased NA, the spherical aberration becomes severer. Thus, the position of the first recording layer 51 is limited within a range from 578 μm to 622 μm inclusive. A distance between the two layers (a thickness of the intermediate layer 55) is set to 20 μm±5 μm (15 μm to 25 μm).
=Existing DVD and Next-Generation DVD=
As described above, there has been proposed a high-capacity HD DVD capable of storing HD images. An HD DVD device (drive or player) which is newly designed for the HD DVD can be designed to read not only HD DVD discs but also DVDs. However, a recording density, a modulation method, signal processing, a track format, and the like of the HD DVD are for the most part different from those of the DVD. Thus, the HD DVD cannot be read by use of a conventional DVD device (drive or player). Specifically, the conventional DVD device has a problem that not only HD movie contents recorded on the HD DVD disc but also conventional DVD movie contents cannot be read. Accordingly, the problem has become a factor that inhibits dissemination of the HD DVD.